1. Field of the Invention
The present invention relates to a method for controlling two-stage combustion furnaces having a first stage operated at a sub-stoichiometric air flow rate and a second stage operated with excess air.
2. Description of the Prior Art
Two-stage combustion is an old art which has found increasing use in the pyro-processing of sewage sludges, solid wastes and other combustible materials. In this process, combustible materials are partially combusted in a first stage to produce combustible gases as well as ash. The gases are consumed in the second stage, often called an afterburner, with an excess of air.
Air is typically supplied to the first stage known as a primary combustion chamber, at a rate which is substoichiometric with respect to the oxygen demand of the combustible material. This is commonly known as starved-air combustion.
A substantial portion of the combustion takes place in the second stage known as the secondary combustion chamber. The combustion is carried out with an excess of air present in order to ensure essentially complete oxidation of the combustible gases and meet government discharge regulations. Secondary combustion chambers are typically operated with air rates of 50-200 percent in excess of the stoichiometric air requirement.
Combustible materials processed in the two-stage furnace are nearly completely gasified to fuel gases and/or oxidized in the primary combustion chamber. The remaining "ash" discharged therefrom is thus composed of primarily inorganic solids.
The term "pyrolysis" is widely used as a synonym for "starved-air" or "two-stage" combustion. Strictly speaking, "pyrolysis" implies heating in the absence of oxygen. Both pyrolytic and oxidative reactions are promoted in the first stage and the second stage is highly oxidative. As already indicated, these two-stage furnaces are typicaly operated with an overall superstoichiometric air rate.
Various types of furnace designs may be used in the two-stage mode; the most popular have a primary combustion chamber of multiple hearth or Herreshoff design.
The control of two-stage combustion systems is typified by U.S. Pat. Nos. 4,013,023 and 4,182,246, in which the quantity of air fed to the hearths of the primary combustion chamber is controlled by hearth temperatures such that the airflow rate is caused to i.e., decrease as hearth temperatures increase. This is called reverse action control. Likewise, the flow rate of auxiliary fuel to burners in the first stage is also decreased to effect a temperature reduction. An oxygen monitor measures residual oxygen in the vapors passing to the second stage and places constraints upon the effect of high or low first-stage temperatures upon regulated air flow and burner operation.
In U.S. Pat. Nos. 4,013,023 and 4,182,246 the air rate and auxiliary fuel rate to the second stage are varied to achieve the desired temperature. As indicated previously, the secondary air rate must be in excess of the stoichiometric requirement for complete combustion in order to meet air pollution standards. This air rate is controlled by the temperature such that air is increased at increasing temperatures in order to quench and cool the burning gases. This is termed direct mode control. An oxygen sensor measures the oxygen concentration in the flue gas from the second stage and increases the air rate thereto whenever the oxygen value falls below a preselected low limit.
For a given furnace processing a given combustible material, a particular adiabatic flame temperature can be achieved at two different air rates, one sub-stoichiometric and one greater than stoichiometric. While a single operating temperature is possible when the airflow is exactly stoichiometric, it is not desirable nor even practical to operate a furnace at that point.
In many two-stage systems, normal variations in feed rate or feed moisture of the combustible materials may temporarily change the first stage from substoichiometric air operation to excess air (superstoichiometric) operation. For example, a sudden increase in feed moisture content may reduce the first-stage temperature to a point at which combustion cannot be maintained, even with the auxiliary burners. Under reverse-action control, the air rate will increase rather than decrease, further cooling the first stage. Thus the controller is incapable of maintaining the desired substoichiometric operation, because there are two possible air rates which may result in the same adiabatic flame temperature. At the indicated temperature the airflow may be either substoichiometric or superstoichiometric.
It is possible to sample the gases from the first stage to determine their combustibles content. This will indicate whether the first stage is operating with substoichiometric airflow. Unfortunately, gases from a sub-stoichiometric combustion chamber also contain tars, oils and soot which tend to foul analytical instruments. These materials may be removed by cumbersome procedures; such cleanup removes combustible matter from the gases and gives erroneous analyses. Determination of oxygen content of the gases leaving the first stage presents similar problems.
U.S. Pat. No. 4,050,389 shows a multiple hearth furnace controlled so that it may continuously change from excess air operation to starved-air operation, and vice versa, as waste material fed to the furnace changes in character.
The principal object of the present invention is to enhance control of a two-stage furnace such that the first stage is always operating in a starved-air mode and the second stage is always operating with excess air, regardless of variations in feed rates and thermal values of the combustible matter.
A further objective is to accomplish the control using only the measurements of i.e., temperature, oxygen, etc. which are already commonly required to perform the temperature control of the individual stages.
A further object is to eliminate the need for direct measurement of oxygen or combustibles content of the gases and vapors passing from the first combustion stage to the second stage. At this point, the gases contain tars, oils and soot which foul analytical instruments unless such materials are previously removed from the gases. When such gases are cleaned by removing combustible solids, the analysis of total combustibles in inaccurate. Likewise, accurate measurement of oxygen concentration in the gases requires cleaning of the gas in a manner which will remove none of the oxygen present.